Direct answer
Unknown means the checker did not get a reliable yes-or-no result. Set these rows aside and recheck later rather than marking them automatically invalid.
More questions and answers
What is a catch-all email address?
A catch-all domain accepts mail for many or all recipient names, so a simple SMTP response cannot prove that a particular inbox exists. Treat it as unconfirmed, not automatically fake.
Should I delete every risky email from my list?
Risky is a broad warning category, not a universal deletion instruction. Inspect the reason, separate those contacts and decide using source, consent and tolerance for bounces.
Why are Yahoo emails marked unknown?
Some receiving systems restrict mailbox probes or return ambiguous answers. Unknown is not proof that a Yahoo mailbox is dead; review and retry based on the verifier's guidance.
Why do corporate emails come back catch-all?
Many businesses configure their mail systems to accept recipient probes without exposing mailbox existence. Verify the domain and use other reliable signals before treating a person-specific address as confirmed.
What is the difference between valid and deliverable?
Vendors use status names differently. A valid format or accepting server does not guarantee the final email will arrive or avoid a spam folder.
What does do-not-mail mean in an email verifier?
It usually indicates an address or category the provider advises excluding. Check the stated reason and preserve your own unsubscribe, complaint and hard-bounce suppression rules.
Can two email checkers disagree about one address?
Yes. Results can change with timeouts, mailbox changes and each provider's methods. Compare the test date and status definitions, and treat uncertain outputs as uncertain.
What is the difference between catch-all and unknown?
Catch-all describes a domain that accepts many recipients; unknown describes a check that could not reach a firm result. Neither alone confirms the named mailbox exists.
Should I recheck an email that was invalid last month?
First look at the original failure. A permanently nonexistent mailbox is different from a temporary network error. Keep hard-bounced contacts suppressed unless there is a documented reason to correct their address.